Generally, in a plasma processing apparatus, plasma of a processing gas is generated within a decompression processing vessel. Further, a thin film is formed on a processing target object within the processing vessel by a gas phase reaction or a surface reaction of radicals or ions included in the generated plasma, or micro-processing such as etching of a material or a thin film on a surface of the processing target object is performed.
For example, a capacitively coupled plasma processing apparatus includes an upper electrode and a lower electrode arranged in parallel to each other within a processing vessel. A processing target object (e.g., a semiconductor wafer, a glass substrate, etc) is mounted on the lower electrode, and a high frequency power having a frequency (typically, 13.56 MHz or higher) suitable for plasma generation is applied to the upper electrode or the lower electrode. Electrons are accelerated in a high frequency electric field generated between the two facing electrodes by applying the high frequency power, and plasma is generated as a result of ionization by collision between the electrons and a processing gas.
Recently, as a design rule is getting more miniaturized in a manufacturing process of a semiconductor device or the like, higher level of dimensional accuracy is required in, especially, the plasma etching. Further, it is required to increase etching selectivity against a mask or an underlying film and to improve etching uniformity in the entire surface of a substrate. For this reason, a pressure and ion energy in a processing region within a chamber tends to be reduced, so that a high frequency power having a high frequency equal to or higher than 40 MHz is used.
However, as the pressure and the ion energy are reduced, an influence of a charging damage, which has been negligible conventionally, can be no more neglected. That is, in a conventional plasma processing apparatus having the high ion energy, no serious problem may occur even when a plasma potential is non-uniform in the entire surface of the substrate. However, if the ion energy is lowered at a lower pressure, the non-uniformity of the plasma potential in the entire surface of the substrate may easily cause the charging damage on a gate oxide film.
In this regard, to solve the above-mentioned problem, a method of pulse-modulating a high frequency power for plasma generation with an on/off (or H level/L level) pulse having a controllable duty ratio (hereinafter, referred to as “first power modulation method”) has been considered effective. According to this first power modulation method, a plasma generation state in which plasma of a processing gas is being generated and a plasma non-generation state in which the plasma is not being generated are alternately repeated at a preset cycle during a plasma etching process. Accordingly, as compared to a typical plasma process in which plasma is continuously generated from the beginning of the process to the end thereof, a time period during which plasma is continuously generated may be shortened. As a result, the amount of electric charges introduced into a processing target object from the plasma at one time or the amount of electric charges accumulated on the surface of the processing target object may be reduced, so that the charging damage is suppressed from being generated. Therefore, a stable plasma process can be performed and reliability of the plasma process can be improved.
Further, conventionally, in the plasma processing apparatus, a RF bias method is widely employed. In this RF bias method, a high frequency power having a relatively low frequency (typically, 13.56 MHz or lower) is applied to the lower electrode on which the processing target object is mounted, and ions in plasma are accelerated and attracted to the processing target object by a negative bias voltage or a sheath voltage generated on the lower electrode. In this way, by accelerating the ions in the plasma and bringing them into collision with the surface of the processing target object, a surface reaction, an anisotropic etching or a film modification may be facilitated.
However, when performing the etching process to form via holes or contact holes by using the plasma etching apparatus, a so-called micro-loading effect may occur. That is, an etching rate may differ depending on the hole size, so that it is difficult to control an etching depth. Especially, the etching rate tends to be higher at a large area such as a guide ring (GR), whereas the etching rate tends to be lower at a small via into which CF-based radicals are difficult to be introduced.
In this regard, to solve the above-stated problem, a method of pulse-modulating a high frequency power used for ion attraction with an on/off (or H level/L level) pulse having a controllable duty ratio (hereinafter, referred to as “second power modulation method”) has been considered effective. According to the second power modulation method, a period during which an on-state (or H-level) of a relatively high power suitable for etching a preset film on the processing target object is maintained and a period during which an off-state (or L-level) of a relatively low power (a high frequency power for ion attraction) suitable for depositing polymer on a preset film on the processing target object is maintained are alternately repeated at a certain cycle. Accordingly, at an area having a larger hole size, a proper polymer layer may be deposited on the preset film at a higher deposition rate, so that the etching may be suppressed. Thus, an undesirable micro-loading effect may be reduced, and it may be possible to perform an etching process with a high selectivity and a high etching rate.
Patent Document 1: Japanese Patent Laid-open Publication No. 2000-071292
Patent Document 2: Japanese Patent Laid-open Publication No. 2012-009544
Patent Document 3: Japanese Patent Laid-open Publication No. 2013-033856
In general, a high frequency power supply provided in a plasma processing apparatus, particularly, a high frequency power supply which applies a high frequency power for plasma generation or a high frequency power for ion attraction into the processing vessel as mentioned above is configured to perform either a control (hereinafter, referred to as “PF control”) under which a power level of the high frequency power outputted therefrom, i.e., a power level of a progressive wave power is maintained constant or a control (hereinafter, referred to as “PL control”) under which a power level of a net input power (hereinafter, referred to as “load power”), which is obtained by subtracting a reflection wave power from the progressive wave power, is maintained constant.
When using the first power modulation method or the second power modulation method in a plasma process, a power of a high frequency power to be pulse-modulated varies in a step-shape between the on-state (or H-level) and the off-state (or L-level) of a modulation pulse, so that a load (plasma) greatly pulsates periodically. Accordingly, in the high frequency power supply that outputs a high frequency power of a continuous wave CW without undergoing the power modulation, neither the PF control nor the PL control may be performed appropriately.
That is, in case of the PF control, as shown in FIG. 14, even if an output of the high frequency power supply, i.e., a progressive wave power PF is maintained constant at a set value PFS, a reflection wave power PR may be varied periodically depending on a variation load (plasma) in synchronization with a modulation pulse, so that a load power PL (PL=PF−PR) is varied periodically. If the load power PL is varied periodically, plasma hunting may easily occur in the high frequency power for plasma generation. Meanwhile, in the high frequency power for ion attraction, ion energy incident on the processing target object may be varied. In any cases, a stable plasma process may not be achieved.
Meanwhile, in the PL control, as depicted in FIG. 15, even if the reflection wave power PR is varied in synchronization with the modulation pulse, it is still possible to maintain the load power PL at a set value PLS by controlling the progressive wave power PF through a feedback route such that the variation of the reflection wave power PR is canceled.
Actually, however, according to the conventional PL control, it is difficult to follow up the periodic variation of the reflection wave power PR and the periodic variation of the progressive wave power PF promptly and effectively through the feedback control. Especially, since the feedback control cannot follow up a rapid load variation that occurs when the modulation pulse is inverted, it is difficult to maintain the load power PL at the set value PLS stably, as depicted in FIG. 16.